1. Field of the Invention
The present invention relates to a weighted surface-wave reflector and, in particular, to a surface wave reflector having metallization strips connected to busbars and other metallization strips not connected to the busbars.
2. Description of the Related Art
Surface-wave arrangements are known which, in addition to at least one interdigital transducer on a piezoelectric substrate, also have at least one reflector for the acoustic wave which is produced electrically in this transducer. This acoustic wave runs in the surface or in the substrate close to the surface of this substrate and is reflected in the reflector, the reflection being a 180.degree. backward reflection in the case of in-line arrangements. Such a reflector comprises, as is known, a large number of strips or fingers which are arranged parallel to one another.
In individual cases, reflectors are required which have a very narrow bandwidth and high selectivity. Such reflectors primarily comprise a very large number of strips which are arranged at a distance from one another, matched to the center frequency. However, surface-wave arrangements having reflectors with a wider bandwidth are also required. This can be achieved with a relatively small number of strips, but with a lower overall reflection capability. Reflectors having a dispersive characteristic are also known.
As a rule, a common feature of all such arrangements is that their characteristics can be disturbed or corrupted by production-dependent scatter or variance, to be precise even if this scatter or variance, for example the center separations, the specified dimensions of width and thickness of the strips and the like, are very small specifically because surface-wave elements are very high precision electronic components.
Surface-wave arrangements having reflectors are used, for example, as resonators. One example of such a resonator is known from European Patent Application 0 257 377, as is illustrated in FIG. 1. A reflector 3 and 4 is located on each of the two sides of the at least one transducer 6, which is located in the center on the surface of the substrate 2.
A particular type of reflector, a so-called "open-short reflector" is known from IEEE Trans. on Ultrasonics Ferroelectrics and Frequency Control, Vol. UFFC -33, No. 4 (1986) pages 369-374. This reflector has strips where the center separation of adjacent strips from one another is one quarter of the wavelength of the acoustic wave. Such a dimension is familiar from split-finger transducers and, based on this, the reflector is also called a "split-finger reflector". The individual strips (frequently also called fingers) are alternately on the one hand metal strips which are short-circuited to one another by busbars and on the other hand electrically "open" strips in the sequence of their side-by-side arrangement (in the x-direction of the main wave propagation). FIG. 2 shows the basic principle of an open-short reflector. Those busbars which electrically connect the metal strips 22 to one another are designated by 21. Between the strips 22, there are strips 23 which are electrically insulated from the strips 22 and from one another. The connection type, open or short-circuited, of the strips 22 and 23 alternates, as can be seen.
As in the case of a split-finger transducer, the mechanical reflections cancel one another out in the case of the open-short reflector in FIG. 2 as well. In contrast, the electrical reflections are amplified because of the alternating connection sequence of the strips 22. This electrical reflection which still occurs is governed essentially by the coupling factor of the material of the substrate, which is a material characteristic.
If a substrate material such as lithium niobate, for example, is used which has a high coupling factor, then an open-short reflector according to FIG. 2 has a high reflection level and a reflector constructed from such groups has a wide bandwidth.
It is unnecessary for the width dimensions of the strips 22 and 23, namely those of the strips 22 which are short-circuited to one another and those of the "open" strips 23, to be of equal size. However, conformity of the quarter-lambda periodicity of the arrangement of the strips 22 is important.
Furthermore, from European Patent Application 0 026 114, it is generally known for surface-wave reflectors to be given a weighting function.